The present invention relates to a fly-by-wire flight control system.
Some fly-by-wire aircraft utilize highly optimized model-following control systems as well as sophisticated electronic mixing. These systems rely on having a correct, real-time mathematical representation of the aircraft available in the flight control system. Small model variations can be accounted for but large variations, such as loss of tail rotor of a rotary-wing aircraft, produce large discrepancies between the flight control system model and the resulting aircraft dynamics. These discrepancies tend to cause difficulties in control of the aircraft after such failures.
In general, there are not many such failures that are survivable in a rotary-wing aircraft—loss of any of the main rotor controls typically results in a complete loss of control. Loss of tail rotor thrust due to loss of the tail rotor drive-shaft or even complete physical loss of the tail rotor, however, can be survivable if the flight control system detects this event and adjusts accordingly.
In mechanically controlled aircraft that are designed to survive such failure events, the burden of detection and control was on the pilot. In a fly-by-wire aircraft, the flight control system must detect such an event and adjust control inputs accordingly; otherwise the aircraft may not be controllable.
Rotary-wing aircraft are typically highly cross coupled and may include a canted tail rotor such that the control mixing algorithm requires the yaw term to feed both pitch and roll axes with a relatively high gain to compensate for the canted tail rotor. During loss of the tail rotor, the aircraft starts to spin and the flight control system responds through application of full yaw input opposite the spin. This typically causes the control mixing algorithm to also apply pitch and roll to compensate for the yaw input, which then results in a relatively large pitch and roll motion of the aircraft since the yaw input did not produce the expected pitch and roll motion response. Such disturbances may further complicate an already difficult loss of tail-rotor event by compounding the yaw motion with pitch and roll motion.
Some aircraft are designed with a very large vertical tail surface such that at cruise speed, the tail rotor needs minimal anti-torque produced thrust. As such, tail rotor failure will not cause a significant change in aircraft behavior unless the conventional fly-by-wire system unintentionally complicates such a disturbance.